Kaitlin Roig-DeBellis, first-grade teacher at Sandy Hook Elementary School, speaks to students at King in Stamford, Conn., on April 23, 2014. Roig-DeBellis is credited with saving the lives of her students during the shooting in 2012 by hiding them in a bathroom. She started the non-profit "Classes 4 Classes," which lets people sponsor educational supplies to classrooms in need. She spoke about her love of teaching, her non-profit, and not letting the tragedy define her.

Kaitlin Roig-DeBellis, first-grade teacher at Sandy Hook Elementary School, speaks to students at King in Stamford, Conn., on April 23, 2014. Roig-DeBellis is credited with saving the lives of her students during the shooting in 2012 by hiding them in a bathroom. She started the non-profit "Classes 4 Classes," which lets people sponsor educational supplies to classrooms in need. She spoke about her love of teaching, her non-profit, and not letting the tragedy define her.

King students listen to Kaitlin Roig-DeBellis, first-grade teacher at Sandy Hook Elementary School, as she speaks at their school in Stamford, Conn., on April 23, 2014. Roig-DeBellis is credited with saving the lives of her students during the shooting in 2012 by hiding them in a bathroom. She started the non-profit "Classes 4 Classes," which lets people sponsor educational supplies to classrooms in need. She spoke about her love of teaching, her non-profit, and not letting the tragedy define her.

Kaitlin Roig-DeBellis, first-grade teacher at Sandy Hook Elementary School, speaks to students at King in Stamford, Conn., on April 23, 2014. Roig-DeBellis is credited with saving the lives of her students during the shooting in 2012 by hiding them in a bathroom. She started the non-profit "Classes 4 Classes," which lets people sponsor educational supplies to classrooms in need. She spoke about her love of teaching, her non-profit, and not letting the tragedy define her.

Kaitlin Roig-DeBellis, first-grade teacher at Sandy Hook Elementary School, speaks to students at King in Stamford, Conn., on April 23, 2014. Roig-DeBellis is credited with saving the lives of her students during the shooting in 2012 by hiding them in a bathroom. She started the non-profit "Classes 4 Classes," which lets people sponsor educational supplies to classrooms in need. She spoke about her love of teaching, her non-profit, and not letting the tragedy define her.

Kaitlin Roig-DeBellis, first-grade teacher at Sandy Hook Elementary School, speaks to students at King in Stamford, Conn., on April 23, 2014. Roig-DeBellis is credited with saving the lives of her students during the shooting in 2012 by hiding them in a bathroom. She started the non-profit "Classes 4 Classes," which lets people sponsor educational supplies to classrooms in need. She spoke about her love of teaching, her non-profit, and not letting the tragedy define her.

Relaying her story to a packed auditorium at King school Wednesday morning, the teacher said she knew they were shots, as they were followed by the sounds of shattering glass.

Roig-DeBellis said it is always possible that your life can change in an instant. So you must live your life a certain way, and how you react means everything, she said

"I knew something evil and bad was in close proximity to my students," she said.

Making a split decision, Roig-DeBellis said she immediately got up, closed the door, turned off the lights and told her students to get into the bathroom. The keys to the classroom door were across the room, so she couldn't lock it.

Though the 15 students didn't want to get into the room, which is 3-by-4 feet wide, they eventually agreed. Meanwhile, she said shots were ringing out as if on a battlefield.

Roig-DeBellis piled the students into the tiny space, placing a few on the toilet and one on the toilet paper holder. When they were all inside, she at first couldn't shut the door and had to re-arrange them.

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"We stood in sheer terror, listening to the horror on the other side," she said.

Some students said they want to live to see Christmas, which was just around the corner, while others said they want to see their parents. Roig-DeBellis told them she was lucky to be their teacher and loves all of them. But the students still wanted to get out.

"I said, `There are bad guys out there right now and we have to wait for the good guys to come.' "

Roig-DeBellis said they waited, trying to have faith and hope. She thought of her fiance, the wedding dress she just purchased and the beach on which she was to be married.

"We started to pray and we started to imagine the very best, and the shooting continued," she said.

Soon, Roig-DeBellis said the shooting stopped and an eerie silence fell over the school. While it seemed like they were in the bathroom for hours, she later learned it was only 45 minutes.

When a police officer came to the door, Roig-DeBellis had a student ask who it was. Then she asked to see the officer's badge, but didn't believe it was real. The authorities then unlocked the door and took everyone out one by one.

It was later revealed that 20-year-old Adam Lanza fatally shot 20 students and six staff members at the elementary school. Before coming to the school, Lanza killed his mother at home. After police arrived at the school, he killed himself.

After the attack, Roig-DeBellis said she was left searching for answers to such questions as how will she and her students regain control of their lives and not let this tragedy define them. In the following weeks, she couldn't be in public, couldn't be in the dark and couldn't shower.

By January, Roig-DeBellis said students and staff went to their new school, and there was an outpouring of support, with toys, stuffed animals, cupcakes and more.

"You name it, it was sent to us," she said.

As she surveyed all of the gifts, Roig-DeBellis told her students that when someone does something nice, you have to do something in return. She said they were excited about the process, because giving always makes you feel better.

Roig-DeBellis started the nonprofit organization, Classes4Classes, whose mission is to teach children in kindergarten through eighth grade that their lives are all connected and everyone has the power to create positive change. Students in one class give a gift to another class, anywhere in the country. The receiving class can accept the gift only after it selects a third class to provide with gift, teaching students to "pay it forward."

Roig-DeBellis said when you teach kindness, compassion, love and empathy, there is no room for hate.

"My purpose has always been to help children," said Roig-DeBellis.

She told the audience that she wanted to be a teacher since she was 4 years old and was thrilled when her dream finally came true.

Since the shootings at Sandy Hook, the teacher said she got married, finished the New York City Marathon and chose her path for healing. She showed pictures of all of the people she's met, including President Barack Obama, President Bill Clinton, Lady Gaga, Katie Couric, Diane Sawyer and Anderson Cooper, and the magazines in which she's appeared.

In her final remarks, Roig-DeBellis told the students always to do what they can to help others, and that they can make a change. She said if they go after what they believe to be most important and do their best, they can do anything.

"Always know your purpose so you can be the change," she said.

She received a standing ovation.

"I thought it was definitely an inspiring story of a teacher who became a hero," said Kevin Sonko, a junior at the school, "how she took these experiences and made them global and made a difference."